1. Field of the Invention
This invention relates to the field of growing high quality oxides on the surface of ion implanted polysilicon.
2. Prior Art
In the field of growing oxide layers on the surface of polysilicon for use as an interpoly dielectric, it is desired to grow an oxide layer with improved electrical and physical characteristics. There are three generally followed methods of growing oxides on an ion implanted polysilicon layer having a variety of disadvantages.
In a first method, the polysilicon is deposited by a chemical vapor deposition (CVD) process at a temperature of above approximately 600.degree. and diffusion doped using a gaseous (generally POC1.sub.3) source. The oxide layer is then thermally grown on the film surface. This method suffers from several serious problems. First, the upper surface of the polysilicon film is rough, leading to poor electrical characteristics in the oxide layer grown on the polysilicon film due to localized field enhancement. Second, the heavy doping level leads to dopant segregtion at the grain boundaries. This leads to non-uniform oxide thickness and results in poor electrical characteristics of the oxide. Third, the quality of the oxide grown on this polysilicon film is poor due to the incorporation of phosphorous from the grain boundaries and results in a low breakdown voltage of the oxide. Fourth, the growth of the polysilicon grains through the subsequent thermal cycles causes residual stress in the oxide which reduces the breakdown voltage further.
A second method of developing the oxide layer is to deposit silicon by a CVD process at a temperature below approximately 580.degree. C. The silicon is then diffusion doped using a gaseous (generally POC1.sub.3) source. The oxide is then thermally grown on the film surface. Although low temperature deposited silicon film is initially smooth, it recrystallizes during the doping cycle resulting in very large grains. The thickness of the oxide layer grown on such a film varies from grain to grain because different crystal faces are exposed to the oxidation ambient by different grains. It is well known that different crystallographic faces oxidize at different rates. The nonuniform oxide thickness results in non-uniform electrical characteristics. In addition, the "steps" caused as a result at grain boundaries can act as charge injection sites.
A third method involves deposition of a silicon film by a CVD process above approximately 600.degree. C. A dopant is then introduced into the film by means of an ion implantation process. The thermal oxide layer is then grown on the surface of this polysilicon film. This method also results in a rough upper surface of the polysilicon film. The oxide grown on it, therefore, has a low breakdown voltage due to localized field enhancement.